1. Field of the Invention
The present invention relates in general to an improved rotary blower with abradable coating for increasing the volumetric efficiency of the rotary blower, and in particular to an abradable coating for a rotary lobe-type pump, compressor, or blower such as a Roots type rotary blower, typically used as an automotive supercharger.
2. Description of the Related Art
Although the present invention may be employed with various types of pumps, blowers, and compressors, such as a screw compressor, it is particularly advantageous when employed with a Roots type blower and will be described specifically in connection therewith, but the present invention is not intended to be limited thereto.
Rotary blowers of the Roots type typically include a pair of meshed, lobed rotors having either straight lobes or lobes with a helical twist with each of the rotors being mounted on a shaft, and each shaft having mounted thereon a timing gear. Rotary blowers, particularly Roots blowers are employed as superchargers for internal combustion engines and normally operate at relatively high speeds, typically in the range of 10,000 to 20,000 revolutions per minute (rpm) for transferring large volumes of a compressible fluid like air, but without compressing the air internally within the blower.
It is desirable that the rotors mesh with each other, to transfer large volumes of air from an inlet port to a higher pressure at the outlet port. Operating clearances to compensate for thermal expansion and/or bending due to loads are intentionally designed for the movement of the parts so that the rotors actually do not touch each other or the housing. Also, it has been the practice to epoxy coat the rotors such that any inadvertent contact does not result in the galling of the rotors or the housing in which they are contained. The designed operating clearances, even though necessary, limit the efficiency of the rotary blower by allowing leakage. This creation of a leakage path reduces the volumetric efficiency of the rotary blower.
In addition to the designed operating clearances limiting the volumetric efficiency of a rotary blower, manufacturing tolerances do exist and can limit the volumetric efficiency. While reducing or even eliminating the manufacturing tolerances can improve the performance and efficiency of the rotary blower, it is not always feasible from a cost perspective.
To enhance pumping efficiency and reduce fluid leakage, it is known to coat one or more of the moving parts of a pump, compressor or rotary blower with a coating material such as a fluoropolymer, for example, as described in U.S. Pat. Nos. 4,806,387 and 4,806,388. While these flexible, thermoplastic type coatings can improve efficiency to some degree, there are still operating clearances which limit the efficiency of the rotary blower.
Still another approach to improving pumping efficiency is the use of a coating with an abradable material. An abradable coating is a material which abrades or erodes away in a controlled manner. An abradable coating is typically employed where there is contact between a moving part and a fixed part, or in some cases where there is contact between two moving parts. As the part moves, a portion of the abradable material will abrade to an extremely close tolerance.
Abradable coatings have found particular application in axial flow gas turbines. The inner surface of the turbine shroud is coated with an abradable material. As the turbine blades rotate, they expand due to generated heat which causes the tips of the blades to contact and wear away the abradable material on the shroud for providing the necessary clearance with a tight seal.
U.S. Pat. Nos. 5,554,020 and 5,638,600 disclose applying an abradable coating to a fluid pump like a rotary blower, compressor, or an oil pump. The abradable coating comprises a polymer resin matrix with solid lubricants having a temperature stability up to 700xc2x0 F. with a nominal coating thickness ranging from 12.5 to 25 microns.
While such coatings have improved the volumetric efficiency of rotary blowers, there still exists a need for an improved rotary blower with an abradable coating that has good adhesion to the rotor, and yet has sufficient lubricity. In addition to having good adhesion to the rotor and sufficient lubricity, the abradable coating should be chemically resistant to automotive related solvents. The lubricating properties of the abradable coating permit a sliding motion between the coated surfaces with a minimum generation of heat while transferring the large volumes of fluid. The abradable coating should still be sufficiently soft so that if any coating abrades away there is little or no contact noise. It is also desirable that the abradable coating be capable of being applied in either a liquid or dry form to the rotors. The abradable coating should significantly increase the volumetric efficiency of a meshed lobed rotary blower by minimizing leakage due to operating clearances.
Accordingly, it is an object of the present invention to provide an improved rotary blower with an abradable coating for increasing the volumetric efficiency of the rotary blower
Another object of the present invention is to provide for the use of an improved abradable coating for a lobed rotor of a rotary blower with a predetermined maximum hardness that has good adhesion to the rotor and sufficient lubricating properties.
Another object of the present invention to provide an improved abradable coating on the lobes of each rotor for providing essentially zero clearance to minimize any leakage therebetween for increasing volumetric efficiency of the rotary blower.
Another object of the present invention is to provide for the use of an improved abradable coating with sufficient lubricating properties to permit a sliding motion between the coated rotors with a minimum generation of heat when transferring large volumes of air.
Still another object of the present invention is to provide for the use of an improved abradable coating for a rotary blower which is sufficiently soft so that if any coating abrades away after a break-in period there is minimal, if any, contact noise.
A further object of the present invention is to provide for the use of an improved abradable coating that can be used for manufacturing an improved Roots type rotary blower in cost-effective, economical manner.
The above and other objects of the present invention are accomplished with the provision of an improved abradable coating on at least a portion of at least one of the lobed rotors in a rotary blower to increase the volumetric efficiency of the rotary blower. The abradable coating comprises a mixture of a coating matrix and a solid lubricant with a maximum hardness value of about 2H on a pencil hardness scale for providing an essentially zero operating clearance for the rotors in the rotary blower. This maximum hardness value achieves a good balance between hardness which offers good adhesion to the rotor and lubricity that permits the sliding motion between the rotors. Preferably, the coating matrix is an epoxy polymer resin in powder form mixed with graphite. The thickness of the abradable coating, prior to the initial break-in, is about 80 to about 130 microns, and preferably about 100 microns.